1. Field of the Invention
The present invention relates to a power transmission device for a front-engine front-drive car, and more particularly, to a power transmission device for a small car wherein an engine is laterally installed in the front part of the car and the power is transmitted to front wheels by means of right and left drive shafts different in length from each other.
2. Description of the Prior Art
There is a steady increase in number of front-engine front-drive type small cars having their engines laterally installed in order to minimize the length of their engine rooms in the front parts of the cars. By the way, it is general that a power transmission device having a transmission, which is directly connected to a crankshaft, integrally assembled on the side of an engine is employed in also the cars of this type. In such cars, it is almost impossible to position the final gear of the counter gear of the transmission on the longitudinal centre plane of the car body. Consequently, a differential gear mechanism to be engaged with the final gear has to be also installed at a position laterally offset from the longitudinal center plane of the car body. Therefore, it is inevitable in such a front-engine front-drive car with its engine laterally installed that there is a difference in length between drive shafts connected between the output shaft of the differential gear mechanism and right and left front wheels respectively.
FIG. 1 is a rear elevation of the front wheel part of a conventional car of this type, particularly showing its power transmission device. As shown in the figure, a differential gear mechanism 1 is usually horizontally installed together with an engine 2 and a transmission 3. In addition, an output shaft center line 4 thereof is at a position higher than a common axis 7 of left and right front wheels 5, 6. Therefore, the descent angle .alpha..sub.L of a longer drive shaft 8, i.e., the angle made between the drive shaft 8 and a horizontal plane is smaller than the descent angle .alpha..sub.R of a shorter drive shaft 9 as viewed in a rear elevation. It is, however, inevitable that such a difference between the descent angles .alpha..sub.L, .alpha..sub.R of the left and right drive shafts 8, 9 causes a difference to be also produced between the moments M.sub.L, M.sub.R applied to the left and right front wheels 5, 6. The larger the descent angle of the drive shaft, the larger the turning moment. Therefore, M.sub.R becomes larger than M.sub.L, and the steering wheel is undesirably forcedly directed toward the longer drive shaft 8.
On the other hand, FIG. 2 is a plan view of the power transmission device shown in FIG. 1. As shown in the figure, it is usual that the engine 2 and the transmission 3 are disposed in front of the front wheels 5, 6 and the output shaft center line 4 of the differential gear mechanism 1 is in front of the common axis 7 of the left and right front wheels 5, 6. Therefore, the angle .beta..sub.R made between the shorter drive shaft 9 and the axis 7 is larger than the angle .beta..sub.L made between the longer drive shaft 8 and the axis 7 as viewed in a plan view. Consequently, when the steering wheel is turned, there is produced a difference between the angle made between the drive shaft 8 and an axle 12 of the front wheel 5 and the angle made between the drive shaft 9 and an axle 13 of the front wheel 6.
By the way, even speed couplings 14, 15 are provided between the drive shafts 8, 9 and the axles 12, 13 respectively in order to transmit power to the front wheels 5, 6 as well as enable the front wheels 5, 6 to rotate about the kingpin axes 10, 11 so that the car can be steered. Such even speed couplings generally have given allowable ranges in the axis crossing angle respectively. Accordingly, the maximum steering angles of the front wheels 5, 6 are restricted by the ranges within which the even speed couplings 14, 15 allow the axial angles between the drive shafts 8, 9 and the axles 12, 13, i.e., the allowable swing angle ranges of the even speed couplings 14, 15 respectively.
When the steering wheel is turned, there are large changes in the angles made between the drive shafts 8, 9 and the axles 12, 13 respectively as viewed in a plan view. Consequently, when the angle .beta..sub.R made between the right drive shaft 9 and the axis 7 as viewed in a plan view is large as shown in FIG. 2, the allowable swing angle range of the right front wheel 6 becomes small when the steering wheel is turned counterclockwise, so that both the steering angles of the left and right front wheels 5, 6 are restricted within small limits. In other words, it becomes impossible to make the most of the allowable swing angle ranges of the even speed couplings 14, 15 respectively. Consequently, the steering wheel turning angle is small and the minimum turning radius of the car inevitably becomes large.